Soliton transmission requires a stream of periodically-emitted short light pulses. For example, for 10 Gbit/s transmission, the pulses must measure about 20 ps (at half-intensity). Moreover the pulses must be close to the "Fourier transform limit", i.e. the product of pulse duration multiplied by pulse spectrum width must be less than a limit of about 0.7.
In a first known method, such pulses can be generated by mode coupling in a semiconductor laser having an external cavity. That method is described by D. M. Bird, R. M. Fatah, M. K. Cox, P. D. Constantine . . . , Electronics letter 26, p. 2086 (1990). A second known method uses gain switching in a monolithic semiconductor laser and is described by L. M. Downey, J. F. Bowers, R. S. Tucker, and E. Agyekum, IEEE, J. Quant. Elec. QE 23, 1039 (1987).
The first method requires a complex laser which is difficult to use, and which is therefore unsuitable for use on site. The second method makes it possible to obtain pulses that are short, but they are very remote from the Fourier limit. Recompression requires a fiber which has abnormal dispersion and which is very long (several kilometers), and there is no certainty that the pulse that is finally obtained will satisfy the conditions for soliton propagation. However, spectral filtering enables satisfactory pulses to be obtained, as indicated by M. Nakazama, K. Suzuki, and Y. Kimura, Optics Letters 15, p. 715 (1990). But that requires critical centering of the filtering device.